1. Field of the Invention
The present invention relates to a method for introducing a fertility restorer gene (Rf gene) of a Raphanus plant into a Brassica plant by means of cell fusion or intergeneric cross and a Brassica plant produced thereby. It also relates to a method for producing a commercial first filial generation (F.sub.1) hybrid of a rapeseed plant utilizing a Kosena-radish-derived cytoplasmic male sterility gene and a fertility restorer gene derived from any of the radish plants and an F.sub.1 hybrid plant of a rapeseed plant produced thereby.
2. Description of Related Art
Breeding of rapeseed plants has been centered on open-pollinated seeds by taking advantage of high self-compatibility affinity of said plants. Breeding of an F.sub.1 hybrid showing a remarkable heterosis has been practically employed in numerous plants. This method has been applied to the breeding of rapeseed plants too. An F.sub.1 plant often shows advantages due to the expression of heterosis. Thus, it has excellent agricultural characteristics such as yield and disease resistance and the breed grown therefrom has relatively uniform characteristics. Further, benefits of a breeder can be protected because of the segregation of generic characteristics in the following generation.
Breeding of F.sub.1 hybrid of rapeseed plants has been attempted by utilizing self-incompatibility or cytoplasmic male sterility (hereinafter, referred to as CMS). The method involving CMS is considered to be more hopeful in terms of efficiency of seed production and facility of maintenance of mating parents.
The only CMS used in the commercial production of F.sub.1 hybrid of rapeseed plants was that derived from a rapeseed plant Polima. The Polima-derived CMS, however, has problems such as instability of male sterility under high temperature. Thus, the pollen fertility can be restored depending on temperature of the flowering period. As a result, the F.sub.1 hybrid lack in homogeneities of the typical F.sub.1 hybrid. In some cases where incompatibility between certain nuclear background and Polima CMS has occurred, the resulting F.sub.1 hybrid plants may be small in size and do not express the heterosis which is one of the most important characteristics of the F.sub.1 hybrids.
The present inventors have previously disclosed a method for introducing cytoplasmic gene of radish directly from its protoplast to a protoplast of Brassica plant such as rapeseed, which method comprises fusing radish-derived protoplast previously irradiated with X-ray with Brassica-derived protoplast previously treated with iodinated compound (Japanese Patent Publication (KOKAI) No. 218530/1989). Also disclosed has been a method for introducing a cytoplasmic gene of radish directly from its cytoplast to a protoplast of Brassica plant such as rapeseed, which method comprises fusing radish-derived cytoplast with Brassica-derived protoplast previously treated with iodinated compound (Japanese Patent Publication (KOKAI) No. 303426/1990). To produce an F.sub.1 hybrid seed, it is necessary that either of the parents has a fertility restorer gene (Rf gene). In our previous methods described above, a radish-derived CMS gene can be introduced into rapeseed though, it was also required to introduce radish-derived Rf gene because the rapeseed lacks an Rf gene corresponding to said CMS gene. Introduction of a radish Rf gene into a rapeseed plant can be effected by any of the known methods such as intergeneric cross method. The intergeneric cross method, however, involves various generic or physiological problems such as hybridization incompatibility, lethality of resultant hybrid and the like, and therefore said method is hardly applicable to obtain desired hybrids. Although introduction of a radish Rf gene into a rapeseed plant by means of intergeneric cross has been reported in case of Tokumasu's CMS (W. Paulmann & G. Robbelen, Plant Breeding, 100, 299-309 (1988)) and Ogura's CMS (R. Pellan-delourme et al., 7th International Rapeseed Congress (1987)), they have not been employed practically. As to the introduction of radish gene into rapeseed plant by means of cell fusion, there have been no reports.